An egg is typically soft-cooked in the shell by boiling it for several minutes, three for a very soft egg, ten for a hard egg. A properly cooked soft-boiled egg has a cooked and firm white surrounding a warm but still liquid yoke. If the white or albumen is soft and runny or if the yoke is firm or hard the egg is not acceptable.
In preparing such eggs, typically for the breakfast trade, it is necessary to take several factors into account to achieve the desired degree of doneness. The size of the egg and its temperature are critical since the temperature of the boiling water is basically fixed. Thus the experienced cook knows to cook a large or cold egg somewhat longer and a small or warm egg somewhat less than is usual. It is, of course, impossible to actually check the degree of doneness without opening the egg, so there is considerable opportunity to make a mistake that will not be detected before the egg is actually served.